cAMP content, phosphodiesterase activator and protein kinase activity can be increased in the anterior pituitary of rats following intraperitoneal injection of reserpine, isoniazid or beta-endorphin. The effects of reserpine can be blocked by pargyline (a monoamine oxidase inhibitor). The effect of isoniazid is not abolished by pargyline but is abolished by small doses of muscimol, diazepam and nipecotic acid, all drugs that increase the GABAergic function. Muscimol, diazepam and nipecotic acid are ineffective against reserpine. The effects of beta-endorphin are blocked by naloxone. These experiments suggest that noradrenaline, GABA and endorphin independently participate in hypothalamic regulation of pituitary function. BIBLIOGRAPHIC REFERENCES: Costa, E., Guidotti, A., Uzunov, P. and Zivkovic, B.: Methods to study the in vivo regulation of cyclic nucleotides in pituitary. In Kumar, T.C. and Karger, J. (Eds.): Neuroendocrinology Regulation of Fertility. Switzerland, Basel, 1976,pp. 196-205. Guidotti, A., Naik, S.R. and Kurosawa, A.: Possible role of gamma-aminobutyric acid (GABA) in the regulation of cAMP system in rat anterior pituitary. Psychoneuroendocrinology, 1977, in press.